At present, the total volume of global fluorine products (including inorganic fluorine) is 2.5 million ton, and the total sales is above 21 billion USD. In recent five years, the annual growth rate of fluorine products is around 3.5% globally. There are more than 100 different types of inorganic fluorine products, the total volume is around one million tons, and the total sales is about two billion US dollars, furthermore, more than half of said inorganic fluorine products is used in electronic chemicals, optical materials and catalysts; there are more than 1000 types of fluorine-containing fine chemicals, the total sales is around fifty billion US dollars, which is 70% of the total sales of the fluorine products. In particular, the development of fluorine-containing pesticides is very rapid, several key fluorine/nitrogen-containing heterocyclic sterilization products have been developed based on studies on activities, structures and functional mechanisms.
Among fluorine-nitrogen heteocyclic compounds, 3-fluoroalkyl-1-methylpyrazol-4-carboxylic acid is an important intermediate. For instance, 3-(difluoromethyl)-1-methyl-1H-pyrazol-4-carboxylic acid (CAS: 176969-34-9) is an important intermediate for pesticides. It plays significantly important roles in many newly developed pesticides, for instance, grain fungicide Bixafen developed by Bayer Cropscience AG, new fungicide Fluxapyroxad developed by BASF, Isopyrazam and Sedaxane developed by Syngenta, etc.

Due to the fact that 3-(difluoro)-1-methyl-1H-pyrazol-4-carboxylic acid is a key intermediate used for preparation of said novel amide fungicides, extensive attention is drawn to its synthetic process, and current preparing methods are summarized below:    1. Claisen condensation of difluoroethyl acetate. At present, it is widely used in industrial mass production, and the preparing method was disclosed in patent publication no. WO2009106619 by BASF. The processing procedure is: difluoroethyl acetoacetate is obtained via Claisen condensation of difluoroethyl acetate, subsequently, the obtained undergoes condensation with triethyl orthoformate and forms 4,4-difluoro-2-(ethoxymethylene)-3-oxoethyl butyrate, subsequently, ring closure takes place between said compound and methylhydrazine, in this way, 3-(difluoromethyl)-1-methyl-1H-pyrazol-4-carboxylic ethyl ester (DFMMP) is formed, which then undergoes hydrolysis with NaOH and acidified with HCl, in this way, 3-(difluoromethyl)-1-methyl-1H-pyrazol-4-carboxylic acid (DFPA) is formed. Wherein, the preparation of difluoroethyl acetate has been reported in a number of inventions, it could be prepared from tetrafluoroethylene monomer, wherein tetrafluoroethyl ether intermediate is formed, and then difluoroethyl acetate is obtained via a two-step reaction. It could also be prepared from dichloroethyl acetate, wherein chlorine atom is converted to fluorine atom with the aid of KF. The said preparing route is classic, with relatively high yield, and the resulting production is stable, however, the disadvantages include relatively long synthetic route, and relatively large amount of waste gas, waste water and waste solid being generated.    2. Dimethyl amino ethyl acrylate method. The said method was disclosed in patent publication no. WO2009043444 by Bayer, furthermore, a similar method, in which dimethyl amino group is replaced by cyclohexyl amino group, was disclosed in patent publication no. WO2009133178 by BASF. The processing procedure of said method is as follows: difluoro acetylfluoride gas is introduced to dimethyl amino ethyl acrylate, the obtained intermediate directly reacts with methylhydrazine, the said ring closure results in formation of 3-(difluoromethyl)-1-methyl-1H-pyrazol-4-carboxylic ethyl ester (DFMMP), the said compound undergoes hydrolysis with the presence of NaOH, and then is acidified with the aid of HCl, in this way, 3-(difluoromethyl)-1-methyl-1H-pyrazol-4-carboxylic acid (DFPA) is formed. Wherein, difluoro acetylfluoride gas is obtained via high temperature splitting of tetrafluoro diethyl ether. The design of said route is rather delicate, only a few steps are required and the resulting yield is high. However, the synthetic cost of dimethyl amino ethyl acrylate is relatively high.    3. Difluorochloroacetyl chloride method. The said method was disclosed in patent publication no. WO2012025469 by Solvay. The said processing procedure is as follows: difluorochloroacetyl chloride (CDFAC) is used as a starting material, and reacts with ethenone, and quenched with the aid of ethanol, in this way, difluorochloroacetyl ethyl acetate is formed, subsequently, 3-(difluorochloromethyl)-1-methyl-1H-pyrazol-4-ethyl carboxylate is formed using method similar to said Claisen condensation, the obtained is reduced with the aid of zinc powder or hydrocarbonized with the aid of Pd, leading to formation of 3-(difluoromethyl)-1-methyl-1H-pyrazol-4-carboxylic ethyl ester (DFMMP), the obtained is hydrolyzed with the aid of NaOH and acidified with the presence of HCl, in this way, 3-(difluoromethyl)-1-methyl-1H-pyrazol-4-carboxylic acid (DFPA) is formed. The resulting yield of said method is high, the obtained atom economy is attractive, the required cost is relatively low, and only a small amount of waste gas, waste water and waste solid is generated. However, the disadvantages include: the required processing is rather long, difluorochloroacetyl chloride has to be obtained via photooxidation, the equipment investment is high, and furthermore, one additional step of reduction and chlorine removal is required.    4. Other synthetic methods. 1) patent publication no. EP2008996 disclosed a method, wherein dichloroacetyl chloride, vinyl ether and methylhydrazine react and form 3-(difluoromethyl)-1-methyl-1H-pyrazol-4-carboxylic acid via five-step reaction. Although, the production cost is further controlled, the required reaction conditions are rather harsh, wherein dichloroacetyl chloride and vinyl ether have to be reacted at −40˜−20° C.; the reaction temperature for introducing carboxylic group via catalytic pressurization is 150° C., the pressure of reaction kettle has to be adjusted from time to time, and therefore, it is not easy to operate, furthermore, the obtained isomers are difficult to separate. 2) patent publication no. WO2009000442 disclosed a method, wherein difluoro ethyl acetate is used as starting material, it reacts with hydrazine hydrate and forms hydrazide, subsequently, the obtained undergoes methylation and ring closure with the aid of ethyl propiolate, leading to formation of 3-(difluoromethyl)-1-methyl-1H-pyrazol-4-carboxylic ethyl ester (DFMMP), however, the resulting yield of said method is relatively low, the price of said ethyl propiolate is rather high, and therefore, it is not suitable for industrialization.